longest increasing path

class Solution {
public:
    int longestIncreasingPath(vector<vector<int>>& matrix) {
        int rows = matrix.size();
        if (!rows) return 0;
        int cols = matrix[0].size();
        
        vector<vector<int>> dp(rows, vector<int>(cols, 0));
        std::function<int(int, int)> dfs = [&] (int x, int y) {
            if (dp[x][y]) return dp[x][y];
            vector<vector<int>> dirs = {{-1, 0}, {1, 0}, {0, 1}, {0, -1}};
            for (auto &dir : dirs) {
                int xx = x + dir[0], yy = y + dir[1];
                if (xx < 0 || xx >= rows || yy < 0 || yy >= cols) continue;
                if (matrix[xx][yy] <= matrix[x][y]) continue;
                dp[x][y] = std::max(dp[x][y], dfs(xx, yy));
            }
            return ++dp[x][y];
        };
        
        int ret = 0;
        for (int i = 0; i < rows; ++i) {
            for (int j = 0; j < cols; ++j) {
                ret = std::max(ret, dfs(i, j));
            }
        }
        
        return ret;
    }
};